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How did you use this formula? We don't have the sum of all terms and we don't have n.

Focus on the logic here: Multiples of 5 between 81 and 358 are 85, 90, 95, 100.....350, 355Question is: How many numbers are they?

8590 = 85 + 1*595 = 85 + 2*5100 = 85 + 3*5...355 = 85 + 54*5

Total 55 terms.

To get the 54 above, you would need to do the following: (355 - 85)/5The point is, this is an arithmetic progression.a, a + d, a + 2d, a + 3d ........ a + ndTotal number of terms = n + 1The last term (355) is first term(85) plus n times the common difference(5).355 = 85 + n*5 Solve this to get n as 54Total number of terms = 54 + 1 = 55
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This is an example of a "fence post" problem - in these types of questions, you have to be careful to include the first number in the set and the last number in the set. You'll notice that the answer choices are consecutive integers; if your math is "off", even a little bit, then you'll probably choose one of the wrong answers and not even know it....

There are a couple of different ways to approach this question. Since the numbers are relatively small, you COULD break the range into groups and do the math in pieces....